Stem cells regenerate tissue by dividing asymmetrically, producing more stem cells (self-renewal) as well as differentiating daughters. Although differentiation is usually considered irreversible, there is increasing evidence that the rules of irreversibility can be broken following injury or in cell culture. The conversion of a differentiated cell to a less differentiated cell type, or dedifferentiation, endows certain organisms with remarkable regenerative properties. Despite centuries of investigation, however, dedifferentiation is not understood molecularly. We use Drosophila spermatogenesis as a model stem cell system, since it parallels mammalian spermatogenesis, yet we can precisely locate the sperm-producing spermatogonial stem cells and manipulate their microenvironment (niche) genetically. In this niche, activation of the Janus kinase-Signal Transducer and Activator of Transcription (Jak-STAT) signaling pathway within spermatogonial stem cells prevents differentiation. However, by manipulating Jak-STAT signaling in vivo we have discovered a surprising degree of plasticity in this lineage;differentiating spermatogonia can reverse their path and dedifferentiate into spermatogonial stem cells. Since dedifferentiation may be a general feature of many stem cell systems, we propose to use the powerful tools of Drosophila genetics to study dedifferentiation. We determine if dedifferentiation serves to replace stem cells lost during aging and if it is an exclusive property of spermatogonia of if it is also activated to regenerate somatic stem cells within this niche. We also pursue our preliminary data supporting two genetic approaches to identify factors involved in dedifferentiation. Together, this work will begin to reveal the molecular mechanisms by which differentiating cells can be coaxed to reverse their path and become functional stem cells. This will advance the field of regenerative medicine and also further our understanding of spermatogonial stem cell renewal, a fundamental aspect of male reproduction.